A variety of printers are known as output devices for data processing devices and the like. DC motors that are suitable for small printers are used as the driving source for driving these printers. These printers have structures provided with a paper feeding unit for feeding the printer paper, a printing unit for printing characters onto the printer paper, and a printing mechanism unit that includes a ribbon feeding unit for feeding a ribbon between the printing unit and the printer paper, where the paper feeding unit and the printer mechanism unit are driven by a single DC motor.
Typically, in a printer it is desirable to minimize the difference in speed between the printing speed of the printing mechanism unit and the paper feed speed of the paper feeding unit in order to reduce the printing misalignment. For example, if the paper feed speed by the paper feeding unit is lower than the printing speed up by the printing mechanism unit, then the paper will be fed before an entire line worth of type is prepared in the printing mechanism unit side, which will cause printing misalignment.
Typically small printers omit the mechanism for speed control, miniaturizing the units by sacrificing printing precision, through using only default values for the printing speed in the printing mechanism unit and for the paper feed speed in the paper feeding unit.
FIG. 17 is a schematic diagram for explaining a conventional printer. In FIG. 17A the printer 101 is provided with a printing mechanism unit 102 for performing the printing, and a paper feeding unit 105 for feeding printer paper 120. The printing mechanism unit 102 is provided with a printing unit 103, which has a type roller, and the like, a platen unit 104 which presses against the type roller, with an ink ribbon (not shown) interposed between the type roller and a printer paper 120, and a ribbon feeding unit 111 for feeding an ink ribbon between the printing unit and the printer paper. The paper feeding unit 105 is provided with a paper feeding roller at 105A, where the paper feeding roller 105A is driven rotationally by a DC motor 107 through a transmission mechanism 106, comprising gears, and the like. Moreover, the printing unit 103, the platen unit 104, and the ribbon feeding unit 111 are also driven by the DC motor 107 through the transmission mechanism 106. The DC motor at 107 receives a supply of power from a power supply 108. (See, for example, Patent Document 1.)
Typically, in printers that use DC motors as the driving source, there is a tendency for the rotational speed of the DC motor to gradually increase. This is because, from the perspective of the DC motor, the load decreases with the passage of time, and because of changes in the characteristics of the DC motor.
The load, when viewed from the DC motor, includes the load of the mechanical system parts, such as the transmission mechanism, such as the gears and the like provided between the DC motor and the paper feeder roller, and the friction, etc., in the bearings in the paper feeder roller, and the like. The load of the mechanical system has a tendency to diminish gradually with the passage of time. Because of this, when driven by a DC motor with a constant torque, there will be a tendency for the speed of rotation to increase due to the reduction in the load of the mechanical system parts.
Moreover, the brushes provided in the DC motor are subjected to friction due to the rotation, and a shift in position occurs due to a shift in the contact points, causing the electrical characteristics of the DC motor to change. The changes in the electrical characteristics cause the speed of rotation to increase.
Normally, in small printers, the paper feeding speed in the paper feeding unit is set so that the difference in the rotational speed of the printing mechanism will be within a tolerable range in light of the change in the rotational speed of the paper feeding unit over time in this way. Although the difference in speed due to changes with the passage of time can be in a tolerable range when the paper feeding speed is low, when the paper feeding speed is high it is difficult to keep the difference in speed within the tolerable range. This is because the difference in speed is proportional to the speed, and thus is greater the higher the speed. Consequently, printers that use DC motors as the driving source have a problem in that the rotational speed gradually increases with the passage of time, leading to printing misalignment. In particular, there is a problem in that the printing misalignment is greater when high-speed printing is performed.
Control so as to ensure consistency in the paper feeding speed has been proposed in response to this type of problem that occurs with small printers. FIG. 17B is an example of a structure for controlling the paper feeding to a constant speed. In this example structure, the speed of the paper feeding roller is detected, and feedback control is performed so as to apply to the DC motor an electric current value that is increased or decreased depending on the deviation from a reference value. The rotational speed of the paper feeding roller 105A is detected by an encoder 110, where the voltage that is supplied to a power supply unit 108 is controlled depending on a control unit 109 that is based on the detected signal, thereby maintaining a constant rotational speed for the DC motor 107. (See Patent Document 2.)    Patent Document 1: Japanese Unexamined Patent Application Publication 2005-138408    Patent Document 2: Japanese Unexamined Patent Application Publication 2001-130069    Patent Document 3: Japanese Unexamined Patent Application Publication 2005-35267